In television receivers and other electronic instrumentalities, variable resistance control units are often employed to provide preset resistance adjustments in the circuits. These preset variable resistance controls are used extensively in television receivers, and the adjustments are generally made by a skilled technician at the factory, or are made by servicemen after the television receiver leaves the factory. These controls are used infrequently; and, in fact, often are never touched after the initial adjustment at the factory. Because of this, the preset variable resistance controls or potentiometers are generally mounted in relatively inaccessible locations on the receiver chassis.
The preset variable resistance potentiometers used in television receivers are relatively small in size. Generally, they have a hollow rotatable hub with a screwdriver receiving slot in either end to permit adjustment from either side of the potentiometer, and/or with a hexagonal internal configuration to permit the insertion of an hexagonal wrench or tool for adjustment. Because of the small size and the fact that these controls sometimes are located in a position which requires a relatively long-handled tool to reach them, making accurate adjustments by use of a screwdriver or hexagonal "Allen" wrench is difficult and time consuming. In addition, the removal of the wrench or screwdriver can result in misadjustments caused by slight rotational movements effected when the tool is removed.
Attempts have been made to provide plug-in rotatable shafts which are inserted into the hollow rotatable hubs of the potentiometers and which have an enlarged head to facilitate rotation of the combined shaft and hub. One plug-in shaft of this type is disclosed in U.S. Pat. No. 2,669,634 to Daily et al. The shaft of the Daily patent, however, is subject to the disadvantage that when it is located at a point within the receiver where it cannot be conveniently grasped by the fingers of the person making the adjustment, the adjustment must be made in the conventional manner by a screwdriver operating directly upon the slot in the rotatable hub of the potentiometer. The disadvantages mentioned previously thus are still present and are not overcome. The plug-in shaft of Daily et al does not include a provision for utilizing it to facilitate the guiding of a screwdriver or a similar tool into operating position.
As a consequence, it is desirable to provide a removable plug-in shaft for preset variable resistance potentiometers which facilitates accurate adjustment of the potentiometer resistance and which further may be easily rotated directly or by means of a screwdriver or the like.